Several methods have been developed for cutting down sulfur emissions of combustion plants. The most common method used so far is wet scrubbing in which method the gases are scrubbed with a water suspension of a reagent, such as lime, reacting with, e.g., sulfur oxides. The water suspension is sprayed into a gas flow in a scrubber arranged after a combustor, whereby sulfur is absorbed into the water suspension and sulfur dioxide reacts with lime, forming calcium sulphate or calcium sulphite EQU CaO+SO.sub.2 +1/2O.sub.2 .fwdarw.CaSO.sub.4
or EQU CaO+SO.sub.2 .fwdarw.CaSO.sub.3.
Water suspension is sprayed in such an amount that sulfur compounds thus formed have not enough time to dry, but they are discharged as a slurry from the lower section of the scrubber. The wet scrubbing process is complicated as it requires means for preparing water suspension and means for after-treatment thereof. Furthermore, the method usually requires additional energy for drying the produced slurry in a slurry after-treatment plant. Therefore, the water suspension is usually fed into the system as dry as possible in order to minimize the energy requirement. Due to the considerable amount of water suspension used, the gas may be cooled to a relatively low temperature in the scrubber and, consequently, the gas discharged from the scrubber may cause corrosion and clogging of filters. Further, energy is consumed for reheating the flue gases prior to leading them out of the system. In the wet scrubbing system, the separation degree of, for example, SO.sub.2, is about 95%.
During the last few years, semi-dry scrubbing methods have been developed, in which a fine alkali suspension, e.g., calcium hydroxide suspension is sprayed through nozzles into a hot flue gas flow in a contact reactor where sulfur oxides dissolve in water and, when the suspension dries, are bound to the lime compound. Water is evaporated in the contact reactor so as to form a solid waste, whereby reaction products of, for example, sulfur and lime are readily separable from the gases by means of a filter. It is attempted to maintain the consistency of the calcium hydroxide suspension on such a level that the heat content of the flue gases is sufficient for evaporating the water therefrom. The thick lime suspension, however, easily deposits layers on the reactor walls and especially around the spray nozzles, and may finally clog the nozzles entirely. The reactors have to be dimensioned relatively large for minimizing the drawbacks caused by deposits. Furthermore, as separate equipment is required for the production of lime suspension, a considerable amount of equipment will be needed in the semi-dry scrubbing method as well, and the gas purification will be fairly expensive. A further drawback is the wearing effect of the lime suspension on the nozzles.
The semi-dry scrubbing method is advantageous for the process because the pollutants in the gases may be removed as dry waste. The process has drawbacks of being difficult to control and providing a sulfur absorption below 90%, which is less than in wet scrubbing. A still further drawback is that inexpensive limestone cannot be used in the semi-dry method because it is very slow to react with sulfur. Either calcium oxide or calcium hydroxide, which are much more expensive, have to be used instead. In big combustion plants, the cost of absorbent is remarkable.
Addition of limestone already into the actual combustion or gasification stage has also been suggested. As a result of such addition, limestone is calcined into calcium oxide in accordance with the following reaction EQU CaCO.sub.3 .fwdarw.CaO+CO.sub.2.
Calcium oxide is then capable of reacting already in the combustor with the sulfur oxides formed therein. The reaction takes place as follows: EQU CaO+SO.sub.2 +1/2O.sub.2 &gt;CaSO.sub.4.
When the reactions proceed, calcium sulphate or calcium sulphite layers, however, cover the surface of the calcium oxide particles preventing sulfur from penetrating the particles, thereby slowing down and finally preventing the reactions between sulfur and lime. Thus, lime will not react completely and will not, therefore, be optimally utilized. Many other parameters, such as Ca/S mole ratio, temperature and residence time also affect sulfur absorption.
The closer to the dew point the reactions take place, the higher the reactivity of alkali compounds becomes. Better reactivity is caused by the fact that, in a wetted particle, reactions take place in a water phase as fast ionic reactions. Close to the dew point, the particles stay wetted and the reactivity also remains on a desired level for a longer time. The moistness of the particles is preferably maintained on such a high level that water surrounds the particles, also penetrating them. As the water penetrates the lime particles, the sulphate or sulphite layer deposited on them will be broken, thereby revealing new reactive lime area. Sulfur dioxide contained in the gases dissolves in the water surrounding the particles and reacts with calcium compounds in the liquid phase.
Finnish patent specification 78401 discloses a method in which sulfur dioxide of flue gases is caused to react in a reaction zone and to be thereby transformed into solid sulphates and sulphites separable from the flue gases. The flue gases are conducted into the lower section of a vertical, lengthy contact reactor. Additionally, powdered lime and water are separately brought into the reactor from several points for the sulfur to be absorbed by lime. Flue gas suspensions are discharged from the upper section of the flow-through reactor and are further conducted to a dust separation stage. By feeding the powdered lime and water separately into the reactor, production, treatment, and spraying of a water suspension are avoided. According to the specification, this method, when used in sulfur absorption with calcium oxide, results in about 80% SO.sub.2 reduction with a mole ratio of Ca/S=1.56 and about 90% SO.sub.2 reduction with a mole ratio of Ca/S=2.22. The 98% SO.sub.2 reduction is not achieved until the mole ratio is Ca/S=4. In this method either, the temperature of the flue gas flow must not be allowed to drop optimally close to the dew point as the solids contained in the flue gas suspension then would deposit layers on the walls of the tubes and other equipment, thus causing troubles in dust separation.
European patent specification 0 104 335 discloses another two-phase, semi-dry flue gas purification system. In this method, dry reagent is fed into the flue gases in a contact reactor in a first stage and water or an aqueous solution, to which dissolved reagent has been added, in a second stage. In the first stage, an inactive surface layer is formed on the reagent particles. The layer slows down or prevents reactions between the reagent and, e.g., sulfur oxide. By adding water in the second stage, the reagent is reactivated. In this manner, the reagent is utilized more completely. The gas temperature is allowed to decrease to a level on which it always stays above the dew point, for example, to 105.degree. C. The gas temperature must not, in this method either, be allowed to decrease too close to the dew point because any wetted particles possibly formed would cause difficulties in the long run, even if the reactivity of the reagent at a lower temperature would be much better. According to the method, the required amount of reagent may be reduced by recycling reagent-containing solid material which has been separated from the gas at a later stage and then regenerated by either grinding or some other way. A drawback of this method is, however, separate equipment needed for handling and storing of the recycled solids.
U.S. Pat. No. 4,509,049 suggests a dry gas purification system in which lime is added to flue gases in a boiler and the lime is then allowed to react with the flue gases in a reactor. The lime, which has partly reacted with the pollutants in the flue gases, is separated from the gases in a filter in the upper section of the reactor. The dry lime thus separated from the gases is collected and ground and then treated with dry steam in order to increase the reactivity of the dry lime, whereafter the lime is recycled into the gas flow at a location prior to the reactor. The dry steam treatment of lime takes 2 to 24 hours, which is a long time involving high consumption of energy.
An object of the present invention is to provide an improved method of purification of waste gases containing, e.g., sulfur, chlorine and fluorine compounds or other condensable compounds.
Another object of the invention is to provide a method by which, e.g., sulfur reduction can be considerably improved, preferably even so that the amount of the reagent need not be increased.
A further object of the invention is to provide a method by which a gas to be purified may be wetted very close to the dew point, for example, 0.degree.-20.degree. C. therefrom, in a wetting reactor, the method still allowing the particles separated from the gases to be removed in a dry state in the wetting reactor.